This invention relates to optical waveguides and more particularly to an acoustooptic filter or tunable coupler between optical waveguides.
Channel and branching waveguides have become important as a means of directing light in integrated optical circuits as well as in an electrooptic material if configured correctly. Examples of such guides are channel guides. In a channel guide light is confined in the vicinity of the channel. In order to form a modulator two channel guides must be brought in close proximity to each other so that the evanescent fields from the two guides overlap. In the overlay region, if an electric field is applied in a guide which exhibits an electrooptic effect, the power may be switched from one guide to the second guide. The principle of operation of such a device has been set forth in an article "Integrated Optics: An Introduction" by Steward E. Miller, The Bell Technical Journal, No. 48, pp. 2059-2069, Sept. 1969.
An electrooptic amplitude modulator using the branching guide has been set forth in patent application Ser. No. 553,053 filed Feb. 25, 1975 now U.S. Pat. No. 3,957,340. Both the channel and branching type modulators require that the substrate and/or film exhibit the electrooptic effect such as LiNbO.sub.3, or quartz on LiTaO.sub.3. Fabrication of waveguides using these materials presents difficulties particularly for the electrooptic films because they must be single crystals and single growth is hard to achieve. Waveguide fabrication involving metal indiffusion into ferroelectric substrates offer an alternate means of realizing electrooptic thin film guides. For waveguides using an electrooptic substrate the electric and optical field penetration into the substrate is small and reduces the figure of merit of such devices. This is not a serious problem however for indiffused guides. Even though these difficulties are present, the electrooptic modulator is an important element in many optical circuits. Further mode conversion behavior of branching or separating planar dielectric waveguides has been set forth in "Mode Conversion in Planar-Dielectric Separating Waveguides" by William K. Burns and A. Fenner Milton, IEEE Journal of Quantum Electronics Vol. QE-11, No. 1, Jan. 1975, pp. 32-39.